SHERATAN (Beta Arietis). The most prominent part of Aries, the Ram, historically the "first"
constellation of the Zodiac (as it held the Vernal Equinox in
ancient times), is a thin flat triangle of stars that Bayer
lettered (from east to west) Alpha (Hamal), Beta (our Sheratan), and Gamma
(Mesarthim). The first two are also in order of brightness, just-
barely-third magnitude (2.64) Sheratan ranking second behind Hamal.
The name originally referred to both Sheratan and Mesarthim, and
evoked "two things" whose natures have been lost to time, the name
now applied to Beta Arietis alone. At first, the star looks like
a very ordinary, white, mid-class A (A5) main sequence dwarf, one
in the normal process of fusing hydrogen to helium in its core. At
a distance of 60 light years, and with a temperature of 8200
Kelvin, it is pumping 22 solar luminosities into space. However,
it keeps a secret from the eye, a companion that has been known for
a century and that is visible only by means Doppler motions in the
spectrum (that detect line of sight movement). While such
discoveries are not at all unusual, Sheratan stands out as a result
of the extremely high eccentricity of the orbit (0.88), the
companion trapped in a record-holding elongated path. Moreover,
the star is an observational treasure. The two stars are so close
together that they cannot be separated directly through the
telescope; all we ever actually see is one star (again common, as
to allow detection via the spectrum requires the stars to be close
and moving quickly). However, sophisticated observation of
Sheratan with an interferometer, a device that makes use of the
interfering properties of light to resolve ultra-fine detail, allow
(as for the brighter component of Mizar)
the pair to be resolved. The masses of the stars (through
gravitational theory) can then be measured with high accuracy.
Averaging 0.64 Astronomical Units apart (89 percent Venus's
distance from the Sun), a star with the mass of the Sun (1.02 solar) orbits a double-solar-mass
(2.00) star every 107 days. Since luminosity is very sensitive to
mass, 95 percent of the light of the system is produced by the
heavier star. The huge eccentricity adds the spice. As they wheel
around each other, the smaller one (undoubtedly a class G star like
the Sun) approaches as close as 0.08 AU (only 20 percent Mercury's
distance from the Sun), and then half an orbit later loops around
at 1.2 AU, 16 times farther away and 20 percent farther than Earth
from the Sun. No close planets could survive the gravitational
onslaught. Such stars, in which the doubling is "visible" by two
techniques (only about 40 are known, Sheratan one of the brighter),
allows accurate assessment of the theoretical relation between
stellar mass and luminosity, and provides powerful evidence that
the theory is correct. The higher mass star will die first. In a
couple billion years, the lower mass G star will be the king of the
pair, while the current luminary will be a shrunken dim white
dwarf.